So-called three-point continuous loop seat belt systems are typically used in cars and trucks in many countries, including the United States. The systems typically include a seat belt retractor, a D-ring, a floor mounting attachment point, and a buckle assembly, each fixed to the vehicle; a length of belt webbing; and a tongue assembly. The belt webbing typically extends from the retractor through the D-ring to an anchor point near the vehicle floor. The tongue assembly is typically slidable along the length of belt webbing between the D-ring and anchor point.
To use the seat belt system, a vehicle occupant typically grasps the tongue assembly, pulling the belt across his/her body, and inserts an end of the tongue assembly into the buckle assembly. When the tongue assembly is fastened to the buckle assembly, a portion of the belt webbing typically extends across the lap of the vehicle occupant. Releasing the tongue assembly from the buckle assembly typically results in the belt webbing being rewound by the retractor.
The tongue assembly needs to slide freely along the belt when the occupant moves the tongue assembly toward the buckle to provide simple and convenient belt length adjustment because not all drivers are the same size, and to compensate for clothing thicknesses such as the use of jackets in the winter. The tongue assembly should also slide along the belt after the occupant unlocks the tongue assembly from the buckle assembly so that the retractor can fully retract the belt. Otherwise, the retractor would carry the tongue assembly to the D-ring, whereupon further movement of the belt would be prevented as the D-ring blocked further movement of the tongue assembly. Free movement of the belt webbing is also critical because tightening of the shoulder belt portion may also lock or tighten the lap belt portion.
In most modern vehicle seat belt systems, the seat belt retractor remains “unlocked.” This permits slack in both the shoulder belt and lap belt portions. The objective of allowing the slack is to permit driver comfort and the ability to reach forward (e.g., to adjust a radio or climate control) without having to loosen or unbuckle the seat belt. However, in the event of many types of accident the seat belt retractor locks, thus reducing further forward motion of the occupant during deceleration. In many modern systems, a “pre-tensioner” mechanism tries to proactively tighten the seat belt upon detection of an impact for faster occupant restraint. The pre-tensioning retraction of the shoulder belt also tightens the lap belt portion because the continuous belt web slides freely through the tongue.
During high performance and off-road driving, loose lap belts allow the driver and passengers to slide in their seats, resulting in loss of optimum vehicle control; loose lap belts allow “slumping” or “slouching,” which can become tiring and induce fatigue during multi-hour trips; and when an infant or child car seat, or booster seat is used, loose lap belts permit unsafe seat movement and potentially dangerous stability situations. In some events, the slack of the typical seat belt design fails to provide a desired degree of controllable restraint in non-accident situations because the lap belt portion of the system is loose.
Most modern seat belt systems include a “pretensioner” that tries to tighten up any slack in the belt webbing in the event of a crash. While the conventional locking system in a retractor keeps the belt from extending any farther, the pretensioner actually pulls on the belt, working together with the locking mechanism in the retractor. Pretensioners are generally wired to the same central control processor that activates the car's air bags. The central control processor monitors motion sensors that respond to the sudden deceleration of an impact, and activates the pretensioner and the air bag after detection of the initiation of an impact.
There is a delay between the moment of impact of a vehicle and the activation of a pretensioner. In this brief interval, the belt webbing may be pulled out of the retractor enough to expose a wearer to greater G-forces (as a result of the occupant slowing down in a shorter distance than if he/she were firmly strapped to the seat), and increase the likelihood of his/her body coming into contact with parts of the car such as the chest on the steering wheel, knees on the instrument panel, hips on the door, or head on the roof.
With the increased number of sport utility vehicles (SUVs) on the roads, with their more top heavy bodies, there has been a large increase in traffic fatalities due to vehicle rollover accidents. Because the forces involved in a rollover are different than those involved in collisions for which seat belt systems were designed, the pretensioners often do not work, leaving a vehicle occupant's body free to move about the interior of the vehicle. Because the ceiling of a vehicle is typically only inches from the occupant's head, the rollover often results in massive head or neck injuries, causing death or paralysis.